A/c Fan Speed

[draider]

Have a well maintained 93 LS400. The A/C fan does not change speeds any longer. Seems to be stuck on low. Cools ok but in Texas heat takes awhile. Saw mention of a temp sensor but do not know it's location or operation. Any suggestions on where to look or what to check would be greatly appreciated. Doug

[obergc]

You didn't say but are lights in the high/med/low speed switches illuminating when you depress them? If you suspect a bad temp sensor in the a/c system (and there are several temp sensors) you should run the a/c diagnostics test by turning on the ignition switch while holding in the AUTO and REC switches on the a/c control assy depressed. After running an indicator test, the diagnostics test will display any fails in the a/c system on the temp indicator. If there are no fails in the a/c system, the temp indicator should indicate 00. If a buzzer sounds when a trouble code is displayed, it means the fail is current, if no buzzer sounds when a trouble code is displayed, it means that the fail is not current but rather a stored code from when a problem was detected previously. Run that test and post any results. The a/c blower fan speed is controlled by a blower resistor for low speed operation, a power transistor for medium speed control and a Extra High Relay for high speed operation. Outputs from the a/c control assy energize the high speed relay and provide control voltage for the med speed power transistor. If there are no control signals from the a/c control assy, the blower would default to a low speed operation. You might check to see that the connectors are firmly seated on the a/c control assy. If they are some, troubleshooting is in order. A reasonably talented tech should be able to find the cause of your problem fairly quickly.

[draider]

Thanks for the info. The lights do light one after another just like normal, in other words it starts at low and cycles up to high as if the fan speed would increase but it does not. The lights are not blinking they are steady as normal. The relay under the hood is good as are all fuses. Ran the diagonistic and got 00 with no error codes. Am going to reboot the computer but think it still might be something else. Going to pull the cover on the passenger side and see if there is anything to look at. I presume the plugs on the rear of the head unit are ok because it cylces normally. Doug

[Paul Sherman]

Great posting - I'll run the diagnostics on mine, too.

I have a similar issue - lights cycle OK, but only two

speeds (low and med are same output).

How do you reboot the computer?

[obergc]

The wires of interest will be a heavy gage blue colored wire and they will be obvious to you when you pull the lower sub panel on the passenger side. There should be a heavy gage blue/red wire that carries 12V power into the evaporator assy to the hot side of the blower motor and the solid blue wire coming out of the evaporator assy is the return or ground lead. These wires will go into the evaporator assy at the bottom, right side where the blower motor is located. This blue ground is routed through either the blower resistor for low speed, the power transistor for med speed or the high speed relay for high speed.

[draider]

I rebooted the computer by disconnecting the ground lead from the battery for about an hour. Thirty minutes may be sufficient but I like to error on the side of caution. Obergc, you seem to really know your stuff. I found the wires you mentioned yesterday , when I looked under the cover and checked the connection and it was good. Other than that, I don't know what they would tell me, unless a voltage check would reveal something although I would think that since the fan is running a low speed something less that 12V would be present. Is there a test for the relay or resistor? I am kinda stumped since there aren't any error codes. I presume I can get to the connector behind the A/C head by removing the radio. Thanks again for your informed input. Doug

[VBdenny]

There is probably a resistor which controls the speed. My Chevy did the same thing and when I took out the resistor unit, I saw the coils, one of the coils had broken and was touching the other. You won't probably get an error code from this. Sorry, but I have no idea where it might be located on the LS or if it even works that way.

[draider]

Here's an update of the A/C fan problem. Traced the blue wire mentioned in the other post. There was a relay in the circuit to the left of the fan with a black rubber boot covering it. Pulled that part and it had two sets of spades coming from the bottom. I checked continuity through the sets of spades and there wasn't any though the larger of the two. Thinking that I have found the problem ordered the part from Lexus. Part #88630-50010 for about $40. Got it today and no change unfortunately. The is a fan controller for $166. I'm going to check some voltages first this coming weekend but I guess that's my next purchase. Doug

[obergc]

Just got back home, hurricane Dennis made landfall about 10 miles from my home so I evacuated. The relay you mention is most likely the extra high speed relay that is mounted on the evaporator case. In addition to the blue wire, it should have a white/black wire that is tied directly to chassis ground. There should also be a black wire that has 12V battery voltage from a 15A Heater fuse. The last wire should be a green/orange wire and that wire goes to the climate control unit with the low/med/hi switches. The climate control unit should provide a ground for the relay green/orange wire when high speed is selected which will cause the relay to energize which will then give you continuity through the relay contacts which will cause the fan to run high speed. With the ignition switch on, if you manually provide a ground for the terminal with the green/orange wire, the fan should run at high speed. If it does, then you know that the problem is most likely in the climate control unit or the green/orange wire is open between the relay and the climate control unit. I am slightly hesitant to advise anyone to do something like this unless I know tht they are at least familiar with basic electronic troubleshooting because of the potential for doing more harm than good. The fact that you even know what a continuity check is and how to do it tells me that you are somewhat familiar with this type of work so good luck. Be happy to help all that I can. Don't try this if the colors don't match up with what I have stated, this info is right from a 93 LS wiring diagram.

[draider]

Obergc thanks for your reply and info. I have a little electrical knowledge as I was a service manager for a generator shop for five years. I will check all the wiring you suggested. Does your book show a fan controller other than what would come from the head unit itself? There is a small black rectangular box mounted behind the head unit that has some wiring going to it. I presume that's the $166 part that the Lexus part man was referring to. Do you know the number for the heater fuse you made reference to? Thanks again. Doug

[obergc]

Not sure what you mean by fuse number, the 15A heater fuse is located in the fuse block that is to the left of the steering column, under the dash. I wouldn't waste a lot of time heading down that route however since it also powers the heater relay and if that relay wasn't energizing, the blower motor would not run at any speed and additionally, the a/c compressor clutch would not work. The a/c control head controls the fan speed directly, don't see any other components in the circuit between the control head and the components that directly control fan speed. The green/orange wire that connects to the highspeed blower relay goes to pin 20 of the larger of the two connectors that connect to the a/c control head unit. Not sure what the unit located behind the a/c head unit that you refer to is, possibly the max control servo motor, it's connectors seem to be located behind the a/c head connectors in the drawing that shows the various connectors in the a/c system.

[draider]

Just a short followup. Finally got around to tracing the wiring suggested in the other posts. Ran a ground wire to the green/orange wire at the relay next to the fan on the passenger side of the car. Relay tripped and fan went to high speed. Traced the wire back to the connection coming out of the back of the control head in the dashboard. Applied 12V ground to that connection(green/orange wire) and fan went to high speed letting me know that wiring was ok and coming to conclusion that the trouble was inside the control board located in the dash. There looks to be a circuit board in the plastic box behind the buttons on the dash. I don't know what parts might be available or if that is a single unit. Anyone had the experience of replacing those parts? For the moment, ran a ground wire to the relay and mounted a toggle switch under dash on drivers side. Therefore have high and low speed fan which is better than what I had. Thanks. Doug

[shm1]

my 94 ls400 blower fan makes noise when on high speed. how do i get to the fan t0 clean it thanks